1. Field of the Invention
The present invention relates to a process for preparing an aqueous polyurethane emulsion, and more particularly, to a process for preparing a homogeneous and stable self-emulsified polyurethane emulsion.
2. Description of the Prior Art
Various processes are known for preparing polyurethane emulsions, for example, by preparing a urethane prepolymer having terminal isocyanate groups from a polyhydroxyl compound and an excess of an organic polyisocyanate, chain-extending said urethane prepolymer using a chain extender which contains a tertiary amino group thereby to obtain a polyurethane composition containing a tertiary amino group, and quaternizing said tertiary amino group, followed by mixing with water, or by mixing with an aqueous solution of acid, in order to prepare a so-called self-emulsified polyurethane emulsion which does not contain an emulsifier.
Another process of preparing a polyurethane emulsion consists of, for example, dispersing a polyurethane composition in water in the presence of an emulsifier.
Polyurethane resins, in general, have excellent physical and chemical properties. But the polyurethane resin films obtained from a polyurethane emulsion prepared by the aforesaid conventional processes exhibit relatively poor physical and chemical properties, as compared to the crosslinked polyurethane resins, because it is difficult to effect chemical cross-linking of the polymers, and the emulsifier remaining in the polyurethane resin causes the resin to exhibit poor properties.
Polyurethane polymers having firm crosslinking usually are very difficult to handle in the preparation steps because of their non-melting and non-soluble properties, and in many cases, handling is impossible. Therefore, it is almost impossible to prepare emulsions by dispersing such polymers in water. Even if they are dispersed in water, the polyurethane emulsions thus prepared often tend to be very unstable.